The invention further relates to a device for detecting a jam during the adjustment of a component of a motor vehicle with the aid of an adjustment drive, comprising means of acquiring measured values related to the adjustment force, which are applied to a comparison unit in order to compare them with a threshold value and, if the threshold value is exceeded, to slow or stop, optionally reverse the adjusting movement.
In the case of the adjustment of components in motor vehicles, such as in particular electrically operated window lifts or sliding roofs, said adjustment being actuated by an external force, an anti-jam system is specified for limiting the excess force generated by the adjustment drive. Ideally, this anti-jam system immediately detects a jam in that during an adjusting operation it compares adjustment force values, which are actually determined in preset positions, with a preset threshold value. If the adjustment force summoned up by the motor rises above this threshold value, this indicates an obstacle in the adjustment path of the component and a “jam” is inferred; the movement of the component is then as a rule stopped, but optionally the motor may also be reversed.
The anti-jam system is effective during the adjusting operation at least within a sub-region of the adjustment path, for example when a sliding window approaches a preset distance from the closed position, wherein at least in this sub-region of the adjustment path in preset, rapidly succeeding positions actual force values or variables correlated therewith, such as values of the window pane velocity, are determined, and corresponding values are compared as “force values” with the threshold value.
If the threshold value is exceeded, then in the known systems as a result of time delays in the electric circuit, such as for example an operating delay of trigger relays, as well as generally as a result of the mechanical inertia of the adjustment system and also the mechanical play contained therein the adjusting movement is not stopped immediately but carries on for a short time, i.e. the jamming continues, before the adjusting movement is stopped and optionally a reversal of motion is initiated. This ongoing clamping leads, depending on the stiffness of the jammed object (for example the arm of a person), to an increase in the jamming force. This increase in the jamming force naturally depends upon how long the ongoing clamping lasts, and in particular also upon how fast the component, for example the window pane or the sliding roof, was being moved prior to the jam. Accordingly, if the jam detection is effected at a constant force, the resulting clamping force depends also upon the velocity of the adjustment of the component. This variation of the clamping force is naturally undesirable. Ideally, for an optimally tuned system it should be possible to assume constant clamping forces in order, on the one hand, to meet the prevailing safety regulations and, on the other hand, to ensure as large as possible a safety margin with regard to a potentially incorrect stopping or reversing of the movement of the motor vehicle component. However, each fluctuation of the clamping force, as mentioned i.e. also as a function of the velocity of adjustment of the component, reduces this safety margin.
In the past, the reduction of the safety margin was mostly accepted and no measures were taken to compensate the described phenomenon. By way of compensation it has however also already been proposed to adjust the threshold value by an amount that is linearly dependent upon the supply voltage. This change in the threshold value is based on the discovery that the change in the adjusting velocity of the respective motor vehicle component are caused primarily by different vehicle electrical system voltages in the motor vehicle. However, even with this procedure only an unsatisfactory compensation of the clamping force fluctuations is possible since the variation of the adjusting velocity is caused only partly by fluctuations in the vehicle electrical system voltage, i.e. the supply voltage. Other influencing variables on different adjusting velocities are present for example as a result of fluctuations in the sluggishness of the mechanical system that are caused in particular also by temperature fluctuations. Such influencing variables lead to velocity changes to a similarly large extent as the fluctuations of the supply voltage.